This application relates generally to wireless communication and more specifically, but not exclusively, to sharing resources among different antenna units that use a common physical layer identifier.
A wireless communication network may be deployed to provide various types of services (e.g., voice, data, multimedia services, etc.) to users within a coverage area of the network. In some implementations, one or more access points (e.g., corresponding to different cells) provide wireless connectivity for access terminals (e.g., cell phones) that are operating within the coverage of the access point(s).
In some networks, low-power access points (e.g., femto cells) are deployed to supplement conventional network access points (e.g., macro access points). For example, a low-power access point installed in a user's home or in an enterprise environment (e.g., commercial buildings) may provide voice and high speed data service for access terminals supporting cellular radio communication (e.g., CDMA, WCDMA, UMTS, LTE, etc.). In general, these low-power access points provide more robust coverage and higher throughput for access terminals in the vicinity of the low-power access points.
Various types of low-power access points may be employed in a given network. For example, low-power access points may be implemented as or referred to as small cells, femto cells, femto access points, femto nodes, home NodeBs (HNBs), home eNodeBs (HeNBs), access point base stations, pico cells, pico nodes, or micro cells. For convenience, low-power access points may be referred to simply as small cells in the discussion that follows. Thus, it should be appreciated that any discussion related to small cells herein may be equally applicable to low-power access points in general (e.g., to femto cells, micro cells, pico cells, etc.).
Each access point (e.g., macro cell or small cell) in a network may be assigned a physical layer identifier that is used to identify the access point, at least on a local basis. For example, a physical layer identifier may comprise a primary scrambling code (PSC) in UMTS or a physical cell identifier (PCI) in LTE. Other types of physical layer identifiers may be used in other technologies.
Typically, a fixed quantity of physical layer identifiers is defined in a given network. Accordingly, in conventional network planning, a network operator carefully assigns physical layer identifiers to access points to avoid so-called collisions between the physical layer identifiers used by different access points.
Physical layer identifier collision involves a scenario where two or more access points within communications range of an access terminal broadcast reference signals (e.g., pilot signals or beacon signals) that are based on the same physical layer identifier. In this case, the access terminal may not be able to decode the signals since the signals are based on the same physical layer identifier. Such collisions may result in significant interference on a channel, thereby causing potential service disruptions.
Physical layer identifier collision is a common problem for small cells deployed in areas with a high density of users, buildings, conventions centers, or other high user density venues. This problem arises because the number of physical layer identifiers allocated for small cell deployments in such areas (e.g., within a given building) is typically restricted to 4-8 physical layer identifiers. However, a large number of small cells may need to be deployed within an area that has high user density (e.g., enterprise environments, shopping malls, apartment buildings, etc.) to provide enough capacity and/or coverage for users within that area. For example, some building deployments may require 6 to 8 small cells per floor depending on the user traffic profile. Given the restricted set of physical layer identifiers and the high small cell density requirements in some deployments, it may be challenging to avoid physical layer identifier collision when performing physical layer identifier planning. However, reusing the same physical layer identifier at close proximity (e.g., at a low path loss to another small cell reusing the same physical layer identifier) will lead to severe inter-small cell interference, adverse impact on uplink and/or downlink capacity, and adverse impact on user experience (e.g., calls dropped, low throughput, etc.).
Physical layer identifier collision may occur in different communication technologies that use different types of physical layer identifiers. For example, a primary scrambling code (PSC) is a type of physical layer identifier used in Universal Mobile Telecommunications System (UMTS). Thus, UMTS systems may suffer from PSC collision. Other technologies may suffer from other types of physical layer identifier collision. For purposes of illustration, the discussion that follows refers to PSC collision. It should be appreciated that this discussion may be equally applicable to other types of physical layer identifier collision.
An example of a PSC collision scenario for a network 100 that employs discrete HNBs is illustrated in FIG. 1. In this simplified example, the network 100 includes a first HNB 102A, a second HNB 102B, a third HNB 102C, and a fourth HNB 102D. Coverage areas of the first through fourth HNBs are represented by corresponding dashed ovals 104A, 104B, 104C, and 104D. As indicated in FIG. 1, in some cases the coverages of neighboring HNBs overlap.
An access terminal (AT) is able to receive service from a given HNB when the AT is within the coverage of that HNB. In the example of FIG. 1, a first access terminal 106A is within the coverage areas 104A and 104B, while a second access terminal 106B is within the coverage area 104C.
FIG. 1 also illustrates the physical layer identifier, specifically PSC, used by each HNB. The first HNB 102A uses PSC X, the second HNB 102B uses PSC X, the third HNB 102C uses PSC Y, and the fourth HNB 102D uses PSC Z. Accordingly, the first and second HNBs 104A and 104B use the same PSC. Consequently, PSC collision may occur in areas of the network 100 where the coverage of the first HNB 104A overlaps with the coverage of the second HNB 104B. Thus, at its current location, the first AT 106A may experience PSC collision.
To address such physical layer identifier (e.g., PSC) collision, a small cell distributed antenna system (small cell-DAS) may be employed. A small cell-DAS mitigates physical layer identifier collision by expanding the foot print of a small cell. Thus, a given geographical area can be covered using a smaller number of physical layer identifiers. Accordingly, in environments with a high concentration of users, deployment of small cells coupled with DAS to cover a large floor space may be advantageous.
FIG. 2 illustrates an example of a HNB-DAS 200 that includes an expansion unit 202 with a first HNB 204A that uses PSC X and a second HNB 204B that uses PSC Y. Each HNB employs several remote antenna units (RAUs) to provide expanded cell coverage. Specifically, the first HNB 204A transmits radiofrequency (RF) signals based on the PSC X to each of a first RAU 206A, a second RAU 206B, and a third RAU 206C. Similarly, the second HNB 204B transmits RF signals based on the PSC Y to each of a fourth RAU 206D and a fifth RAU 206E. Each RAU, in turn, includes an RF amplifier (not shown) and at least one antenna (A) for transmitting these RF signals.
Respective coverage areas 208A, 208B, 208C, 208D, and 208E of the first through fifth RAUs 206A-206E are represented by dashed ovals. Thus, the first HNB 204A provides service over the coverage areas 208A, 208B, and 208C, while the second HNB 204B provides service over the coverage areas 208D and 208E. Accordingly, in comparison to the HNBs of FIG. 1, each HNB of FIG. 2 provides a larger coverage area.
Moreover, the PSC X is used in the coverage areas 208A, 208B, and 208C, while the PSC Y is used in the coverage areas 208D and 208E. Consequently, fewer PSCs need be deployed over a given geographical area in comparison to the HNB architecture of FIG. 1. Of note, there will not be PSC confusion between the coverage areas 208A-208C even though the same PSC is used in the coverage areas since these coverage areas are all associated with the first HNB 204A.
The use of a small cell-DAS may lead to certain inefficiencies relating to system capacity, however. In general, small cells have limited capacity in terms of the number of concurrent users supported. For example, some types of small cells may support a maximum of 15 simultaneous users. Such hardware limitations may reduce the usability of small cells in areas having a high concentration of users (e.g., enterprise buildings, shopping malls, hospitals, etc.). In particular, the limited user capacity of small cells may not match the air interface capacity of the wireless network. For example, the 3GPP UMTS standard specifies that the maximum number of users supported by an access point can be in excess of 60. Accordingly, the use of a small cell-DAS for serving a large building may result in insufficient user capacity in the uplink and/or the downlink.